The present invention relates to a stylus driving apparatus for printers and more particularly to an improvement of a stylus driving apparatus of an impact dot line printer.
Generally, a dot line printer has an advantage that such a printer having a wide print width enables line printing at a higher speed than a serial printer. Recently, demand for a dot line printer having high speed printing performance has increased. In order to increase the printing speed, it is necessary to reduce a printing impact period and decrease the space between print wires disposed on a printing line. Further, an impact dot line printer having very fine stylus is popular, because such a printer can form a dot matrix character having thin portions similar to a character printed by type. When the impact speed to a record member is increased, the impact force by the fine stylus becomes large which deteriorates an inked ribbon or the record member. In order to eliminate such a disadvantage, the equivalent mass of a stylus portion must be restrained to be as small as possible.
FIG.5 shows a conventional stylus driving apparatus which is disclosed by U.S. Pat. No. 3,941,051. The apparatus comprises a leaf spring 102 made of magnetic material, a base disk 103 secured to an end of the leaf spring 102, and a stylus 104 secured to the base disk 103. The leaf spring 102 is secured between a base plate 116 and a holding plate 105 by a screw 106. The base plate 116 is secured to a side of a permanent magnet 108. A yoke 107 made of magnetic material is secured to the other side of the permanent magnet 108. A core 110 made of magnetic material and having a coil 111 is secured to the yoke 107 at an end thereof. The other end of the core 110 corresponds to the other end of the leaf spring 102 having the base disk 103 and is adapted to attract the leaf spring 102 by the magnetomotive force of the permanent magnet 108 against the elastic force of the leaf spring 102. The coil 111 is connected to an electric circuit (not shown) and excited to produce a magnetic flux in the direction opposite to the direction of the magnetic flux which is produced by the magnetomotive force of the permanent magnet 108.
In operation, when the coil 111 is excited, the attracting force between the core 110 and the end portion of the leaf spring 102 is cancelled by the magnetomotive force by the excitation, the end of the leaf spring 102 moves to the right by own elastic force. Thus, the stylus 104 is impacted to an inked ribbon (not shown) to print a dot on a recording paper (not shown).
In such a structure, a spring steel having a high spring constant is necessary for the leaf spring 102 in order to reduce the printing impact period. However, in order to attract an end portion of the leaf spring 102 having a high spring constant, it is necessary to increase the areas of the attracting surface of the core 110 and of the attracted surface of the base disk 103 and to increase the magnetic flux density of that portion. Therefore, the permanent magnet 108 must have a large magnetomotive force and the yoke 107, core 110 and leaf spring 102 should be made of a material having a high permeability and should have large sectional areas for flux, respectively.
It is also desirable to make the leaf spring out of spring steel, since the spring steel has a high durability for bending moment. However, the spring steel has a low permeability. In addition, the leaf spring must be made with a small width in order to decrease the distance between stylus driving units along the printing line, which results in decrease of the sectional area. As a result, magnetic flux cannot be sufficiently applied to the attracting portion between the core 110 and the end of the leaf spring 102. Therefore, the attracting force exerted on the armature is restricted to a small value, which does not permit an increase of the spring constant of the leaf spring. Thus, the printing impact period can not be reduced.
U.S. Pat. No. 4,044,668 discloses a line printer, in which the front face of a housing surrounding a hammer mechanism is made of magnetic material, forming an additional flux path in parallel to the path defined by a leaf spring of the hammer mechanism. This additional flux path provides for increased flux density for the attracting portion, providing a greater attracting force for the armature. However, the armature is positioned in a position remote from the additional path when the armature is attracted to a magnet. Therefore, the attracting force is not made large so as to enable an increase in the spring constant of the leaf spring in order to reduce the printing impact period.